Hypex Amp, using speaker level outputs as Input... (BD-Design)

I have the Hypex PSC2.400 and bridged mono and used solely for the woofer section. My speakers are 104db efficient. There is a little hum, not overbearing, but some (more than the amps that are driving the Hypex. I am wondering if bridging the amp imposing more hum than operating just one channel. I think having 400w into 4 ohms is adequate and do not need to bridge. What do you think?